1. Field of the Invention
The present invention relates to a stereoscopic image display apparatus, for example, to a stereoscopic image display apparatus used as a head mounted display (HMD) and enabling a stereoscopic image to be viewed as a virtual image.
2. Description of the Prior Art
A conventional HMD is designed so that virtual images for the right eye and for the left eye are projected at a predetermined distance and a stereoscopic image is displayed based on the binocular parallax. According to this HMD, since the focusing function of the human eye is ignored, the eyes are fatigued if a stereoscopic image is viewed for a long time. A stereoscopic image display apparatus intended to solve this problem is disclosed in Japanese Laid-open Patent Application No. H6-235885. In this apparatus, the directions of the lines of sight of both eyes are detected to thereby obtain the convergence position of the eyes and a virtual image is projected onto the convergence position of the eyes.
As another apparatus for displaying a stereoscopic image, an HMD of a laser scanning type (which displays images directly on the retina) is disclosed in U.S. Pat. No. 5,355,181. In this apparatus, a stereoscopic image is displayed by performing focusing at high speed based on distance data concerning each pixel of the image while performing laser scanning.
The apparatus disclosed in Japanese Laid-open Patent Application No. H6-235885 employs a principle that the imaging position is caused to coincide with the convergence position of the eyes by automatically adjusting the imaging position of the displayed image in accordance with variation in convergence position of the left and right eyes. However, since the pupil distance varies among people, it is difficult to obtain the convergence position of the viewer's eyes with accuracy. If the imaging position of the displayed image is inaccurately adjusted, the convergence position of the eyes and the imaging position do not coincide with each other, so that the eyes are apt to get fatigued.
When a close object placed before a far background is viewed through a stereoscopic image display apparatus, it is desirable that the edge of the clear-cut object placed in the foreground should be clearly seen in front of the blurred background. However, according to the apparatus disclosed in U.S. Pat. No. 5,355,181, the image of the blurred background enters the edge of the object placed in the foreground, so that an overlapping image is viewed. For this reason, the displayed stereoscopic image is unnatural and the reality is impaired.